We were learning form Coussot et al., Oil & Gas Science and Technology—Rev. IFP, Vol. 59 (2004), No. 1, pp. 23-29, that drilling muds are very complex fluids used to drill oil wells; their functions are various: to carry the rock cuttings to the surface, to maintain a sufficient pressure against the rock formation, to lubricate and cool the bit. There are a few families of drilling muds: oil based muds (invert emulsion of brine into an oil phase with various additives) and water based muds (aqueous solutions of clays and polymers). Originally prepared from produced oil, oil based muds formulations have evolved to very complex compositions of various additives. The base oil may be of various nature, and additives are very complex: water droplets, surfactants, organophilic clays, viscosifyers, various solids and others. These additives give specific properties to the mud, particularly regarding rheological properties. Drilling muds are often described as thixotropic shear thinning fluids with a yield stress. Due to their complex composition, drilling muds exhibit an internal structure which is liable to modify according to the flowing and shear conditions, which may lead to non-homogenous phenomena. It is therefore important to develop investigation techniques allowing visualizing the internal structure of the fluid in parallel to rheological measurements.
Coussot et al further presented rheological experiments coupled to magnetic resonance imaging (MRI). Using this technique, they have determined the velocity profile in a viscometric flow. Coussot et al did not disclose or taught use of MRI in returning mud treatment, as be disclosed below.
More about the aforesaid drilling muds: water-based mud (WBM) is a most basic water-based mud system begins with water, then clays and other chemicals are incorporated into the water to create a homogenous blend resembling something between chocolate milk and a malt (depending on viscosity). The clay (called “shale” in its rock form) is usually a combination of native clays that are suspended in the fluid while drilling, or specific types of clay that are processed and sold as additives for the WBM system. The most common of these is bentonite, frequently referred to in the oilfield as “gel”. Gel likely makes reference to the fact that while the fluid is being pumped, it can be very thin and free-flowing (like chocolate milk), though when pumping is stopped, the static fluid builds a “gel” structure that resists flow. When an adequate pumping force is applied to “break the gel”, flow resumes and the fluid returns to its previously free-flowing state. Many other chemicals (e.g. potassium formate) are added to a WBM system to achieve various effects, including: viscosity control, shale stability, enhance drilling rate of penetration, cooling and lubricating of equipment. Oil-based mud (OBM) can be a mud where the base fluid is a petroleum product such as diesel fuel. Oil-based muds are used for many reasons, some being increased lubricity, enhanced shale inhibition, and greater cleaning abilities with less viscosity. Oil-based muds also withstand greater heat without breaking down. The use of oil-based muds has special considerations. These include cost, environmental considerations such as disposal of cuttings in an appropriate place to isolate possible environmental contamination and the exploratory disadvantages of using oil based mud, especially in wildcat wells due inability to analyze oil shows in cuttings, because the oil based mud has fluorescence confusing with the original oil of formation. Therefore induces contamination of cuttings samples, cores, sidewall cores for geochemical analysis of TOC and masks the real determination of API gravity due to this contamination. Synthetic-based fluid (SBM) is a mud where the base fluid is a synthetic oil. This is most often used on offshore rigs because it has the properties of an oil-based mud, but the toxicity of the fluid fumes are much less than an oil-based fluid.
On a drilling rig, mud is pumped from the mud pits through the drill string where it sprays out of nozzles on the drill bit, cleaning and cooling the drill bit in the process. The mud then carries the crushed or cut rock (“cuttings”) up the annular space (“annulus”) between the drill string and the sides of the hole being drilled, up through the surface casing, where it emerges back at the surface. Cuttings are then filtered out with either a shale shaker, or the newer shale conveyor technology, and the mud returns to the mud pits. The mud pits let the drilled “fines” settle; the pits are also where the fluid is treated by adding chemicals and other substances.
The returning mud can contain natural gases or other flammable materials which will collect in and around the shale shaker/conveyor area or in other work areas. Because of the risk of a fire or an explosion if they ignite, special monitoring sensors and explosion-proof certified equipment is commonly installed, and workers are advised to take safety precautions. The mud is then pumped back down the hole and further re-circulated. After testing, the mud is treated periodically in the mud pits to ensure properties which optimize and improve drilling efficiency, borehole stability, and other requirements listed below.
Drilling muds are classified based on their fluid phase, alkalinity, dispersion and the type of chemicals used. Dispersed systems are Freshwater mud—Low pH mud (7.0-9.5) that includes spud, bentonite, natural, phosphate treated muds, organic mud and organic colloid treated mud. High pH mud example alkaline tannate treated muds are above 9.5 in pH. Water based drilling mud that represses hydration and dispersion of clay—There are 4 types: high pH lime muds, low pH gypsum, seawater and saturated salt water muds. Non-dispersed systems are low solids mud—These muds contain less than 3-6% solids by volume and weight less than 9.5 lbs/gal. Most muds of this type are water-based with varying quantities of bentonite and a polymer. Emulsions usually selected from oil in water (oil emulsion muds) and water in oil (invert oil emulsion muds). Oil based muds contain oil as the continuous phase and water as a contaminant, and not an element in the design of the mud. They typically contain less than 5% (by volume) water. Oil-based muds are usually a mixture of diesel fuel and asphalt, however can be based on produced crude oil and mud, see M. G. Prammer, E. Drack, G. et al. 2001. The Magnetic-Resonance While-Drilling Tool: Theory and Operation, SPE Reservoir Evaluation & Engineering 4(4) 72495-PA which is incorporated herein as a reference.
U.S. Pat. No. 6,268,726 to Numar Corporation, named “Method and apparatus for nuclear magnetic resonance measuring while drilling” ('726) discloses an NMR measurement-while-drilling tool having the mechanical strength and measurement sensitivity to perform NMR measurements of an earth formation while drilling a borehole, and a method and apparatus for monitoring the motion of the measuring tool in order to take this motion into account when processing NMR signals from the borehole, is incorporated herein as a reference. US '726 further discloses an apparatus wherein its tool has a permanent magnet with a magnetic field direction substantially perpendicular to the axis of the borehole, a steel collar of a non-magnetic material surrounding the magnet, antenna positioned outside the collar, and a soft magnetic material positioned in a predetermined relationship with the collar and the magnet that helps to shape the magnetic field of the tool. Due to the non-magnetic collar, the tool can withstand the extreme conditions in the borehole environment while the borehole is being drilled. Motion management apparatus and method are employed to identify time periods when the NMR measurements can be taken without the accuracy of the measurement being affected by the motion of the tool or its spatial orientation.
Other patents directed to practical NMR measurements while drilling are U.S. Pat. No. 5,705,927 issued Jan. 6, 1998, to Sezginer et al.; U.S. Pat. No. 5,557,201 issued Sep. 17, 1996, to Kleinberg et al.; and U.S. Pat. No. 5,280,243 issued Jan. 18, 1994, to Miller; U.S. Pat. No. 6,362,619 and U.S. Pat. No. 8,373,412, U.S. Pat. No. 8,143,887 “Apparatus and method for real time and real flow-rate measurement of multi-phase fluids with MRI” by Shell Oil Company (Houston, Tex., herein after '887)—all are incorporated herein as a reference.
An MRI/NMR-based means and methods for real-time in-vivo rheology measurements of drilling muds, especially for optimizing the recycling conditions and treatment of the mud, including continuous, one-step on-line measurement of mud-related parameters is still a long felt need. Moreover, such measuring system for defining mud characteristics, such as its fluid phase, alkalinity, dispersion and the type of chemicals used to be further add is an unmet need, currently necessary for optimizing and improve drilling efficiency, borehole stability, and other requirements as stated above.